Depending on the type and severity of damage, knee pain can be a minor annoyance, causing an occasional twinge when you kneel down or exercise strenuously. Or it can lead to severe discomfort and disability.
Many minor knee problems respond well to self-care measures. More serious injuries, such as a ruptured ligament or tendon, may require surgical repair. But knee surgery can often be performed with a procedure that uses a few small incisions rather than a single large one. Because there's less tissue disruption than with traditional surgery, you're likely to heal more quickly and comfortably.
Still, it's better not to have knee pain at all. Although every knee problem can't be prevented - especially if you're active - you can take certain steps to reduce the risk of injury or disease.
Signs and symptoms


A knee injury can affect any of the ligaments, tendons or fluid-filled sacs (bursa) that surround your knee joint as well as the bones, cartilage and ligaments that form the joint itself. Because of the knee's complexity, the number of structures involved, the amount of use it gets over a lifetime and the range of injuries and diseases that can cause knee pain, the signs and symptoms of knee problems can vary widely.
Acute knee pain
Severe knee pain that comes on suddenly but that's of limited duration (acute pain) is often the result of injury. Some of the more common knee injuries and their signs and symptoms include the following:

        Tendon injuries (tendinitis). Tendinitis is irritation and inflammation of one or more tendons - the thick, fibrous cords that attach muscles to bone. Athletes - especially runners, skiers and cyclists - are prone to develop inflammation in the patellar tendon, which connects the quadriceps muscle on the front of the thigh to the larger lower leg bone (tibia). Tendinitis can occur in one or both knees and often causes pain and swelling at the front of your knee and just below your kneecap. The discomfort usually isn't constant but tends to occur when you jump, run, squat or climb stairs. The quadriceps or patellar tendons may also rupture, either partially or completely. In that case, the pain is likely to be most intense when you try to extend your knee. If the tendon is completely ruptured, you won't be able to extend or straighten your knee at all.
        Meniscus injuries. The meniscus is a C-shaped cartilage that curves within your knee joint. Meniscus injuries involve tears in the cartilage, which can occur in various places and configurations. For example, the cartilage may tear lengthwise or from the inside to the outside rim of the meniscus (radial tear). Although you may not notice small tears, in most cases, you'll have pain and mild to moderate swelling that develops over 24 to 48 hours. Occasionally, a lengthwise tear flips into the knee joint instead of staying around the joint's edge, an injury called a bucket-handle tear. A flap of the torn cartilage can interfere with knee movement and cause your knee joint to lock so that you can't straighten it completely. The pain and swelling often disappear on their own, but they're likely to return when you're active again. In addition, repeated injuries can increase the size and severity of existing tears.
        Bursitis. Some knee injuries cause inflammation in the bursae, the small sacs of fluid that cushion the outside of your knee joint so that tendons and ligaments glide smoothly over the joint. Bursitis can lead to warmth, swelling and redness over the inflamed area, aching or stiffness when you walk, and considerable pain when you kneel. Sometimes the bursa located over your kneecap bone (prepatellar bursa) can become infected, leading to fever, pain and swelling. When the pes anserine bursa on the lower inner side of your knee is affected, you're likely to have pain when you go up or down stairs.
        Loose body. Sometimes injury or degeneration of bone or cartilage can cause a piece of bone or cartilage to break off and float in the joint space. This may not create any problems unless the loose body interferes with knee joint movement - the effect is something like a pencil caught in a door hinge - leading to pain and a locked joint.
        Dislocated kneecap. This occurs when the triangular bone that covers the front of your knee (patella) slips out of place, usually to the outside of your knee. You'll be able to see the dislocation, and your kneecap is likely to move excessively from side to side. You're also likely to have intense pain and swelling in the affected area and difficulty walking or straightening your knee. Unfortunately, once you've had a dislocated kneecap, you're at increased risk of having it happen again. Although you may not experience as much swelling or discomfort with subsequent episodes, repeated dislocations can lead to chronic knee pain. But good rehabilitation, with a focus on strength training of the muscles that control your kneecap, can help prevent dislocation.
        Osgood-Schlatter disease. Primarily affecting athletic teens and preteens, this overuse syndrome causes pain, swelling and tenderness at the bony prominence (tibial tuberosity) just below the kneecap. The pain, which can range from mild to debilitating, is usually worse with activity, especially running and jumping, and improves with rest. Osgood-Schlatter disease frequently affects just one knee but sometimes develops in both knees. The discomfort can last from weeks to months and may continue to recur until your child stops growing.
        Iliotibial band syndrome. This occurs when the ligament that extends from the outside of your pelvic bone to the outside of your tibia (iliotibial band) becomes so tight that it rubs against the outer portion of your femur. Distance runners are especially susceptible to iliotibial band syndrome, which generally causes a sharp, burning pain in the knee that often begins 10 to 15 minutes into a run. Initially, the pain goes away with rest, but in time it may persist when you walk or go up and down stairs.
        Hyperextended knee. In this injury, your knee extends beyond its normally straightened position so that it bends back on itself. Sometimes the damage is relatively minor, with pain and swelling when you try to extend your knee. But a hyperextended knee may also lead to a partial or complete ligament tear, especially in your ACL.
        Septic arthritis. Sometimes your knee joint can become infected, leading to swelling, pain and redness. Septic arthritis often occurs with a fever.

Chronic knee pain
Sometimes an injury can lead to ongoing (chronic) knee pain. Often, however, chronic pain results from a medical condition such as:

        Rheumatoid arthritis. The most debilitating of the more than 100 types of arthritis, rheumatoid arthritis can affect almost any joint in your body, including your knees. In addition to pain and swelling, you're likely to have aching and stiffness, especially when you get up in the morning or after periods of inactivity; loss of motion in your knees and eventually deformity of the knee joints; and sometimes a low-grade fever and a general sense of not feeling well (malaise). Rheumatoid arthritis usually affects both knees at the same time. And although it's a chronic disease, it tends to vary in severity and may even come and go. Periods of increased disease activity - called flare-ups or flares - often alternate with periods of remission.
        Osteoarthritis. Sometimes called degenerative arthritis, this is the most common type of arthritis. It's a wear-and-tear condition that occurs when the cartilage in your knee deteriorates with use and age. Osteoarthritis usually develops gradually and tends to cause varying degrees of pain and swelling when you stand or walk and before a change in the weather. It also can lead to stiffness, especially in the morning and after you've been active, and to a loss of flexibility in your knee joints.
        Gout. With this type of arthritis, you're likely to experience redness, swelling and intense pain in your knees that comes on suddenly - often at night - and without warning. The pain typically lasts five to 10 days and then stops. The discomfort subsides gradually over one to two weeks, leaving your knee joints apparently normal and pain-free. Another condition, pseudogout (chondrocalcinosis), which mainly occurs in older adults, can cause severe inflammation and intermittent attacks of pain and swelling in large joints, especially the knees.
        "Chondromalacia of the patella" or patellofemoral pain. This is a general term that refers to pain arising between your patella and the underlying thighbone (femur). It's common in young women, especially those who have a slight misalignment of the kneecap, in athletes, and in older adults, who usually develop the condition as a result of arthritis of the kneecap. Chondromalacia of the patella causes pain and tenderness in the front of your knee that's worse when you sit for long periods, when you get up from a chair and when you climb stairs. You may also notice a grating or grinding sensation when you extend your knee.

Causes
In the simplest terms, a joint occurs wherever two bones come together. But that definition doesn't begin to convey the complexity of joints, which provide your body with flexibility, support and a wide range of motion.
You have four types of joints: fixed, pivot, ball-and-socket and hinge. Your knees are hinge joints, which, as the name suggests, work much like the hinge of a door, allowing the joint to move backward and forward. Your knees are the largest and heaviest hinge joints in your body. They're also the most complex. In addition to bending and straightening, they twist and rotate. This makes them especially vulnerable to damage, which is why they sustain more injuries on average than other joints.
A closer look at your knees
Your knee joint is essentially four bones held together by ligaments. Your thighbone (femur) makes up the top part of the joint, and two lower leg bones, the tibia and the fibula, comprise the lower part. The fourth bone, the patella, slides in a groove on the end of the femur.
Ligaments are large bands of tissue that connect bones to one another. In the knee joint, four main ligaments link the femur to the tibia and help stabilize your knee as it moves through its arc of motion. These include the collateral ligaments along the inner (medial) and outer (lateral) sides of your knee and the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which cross each other as they stretch diagonally from the bottom of your thighbone to the top of your shinbone.
Other structures in your knee include:

        Tendons. These fibrous bands of tissue connect muscles to bones. Your knee has two important tendons, which make it possible for you to straighten or extend your leg: the quadriceps tendon, which connects the long quadriceps muscle on the front of your thigh to the patella, and the patellar tendon, which connects the patella to the tibia.
        Meniscus. This C-shaped cartilage, which curves around the inside and outside of your knee, cushions your knee joint.
        Bursae. A number of these fluid-filled sacs surround your knee. They help cushion your knee joint so that ligaments and tendons slide across it smoothly.

Normally, all of these structures work together smoothly. But injury and disease can disrupt this balance, resulting in pain, muscle weakness and decreased function.
Knee injuries: The hows and whys
Many knee injuries are due to overuse, problems with alignment, sports or physical activities, and failure to warm up and stretch before exercise. But they can also result from trauma, such as a car accident, a fall or a direct blow to your knee.
Common knee injuries and their causes include:

        Tendon injuries. Inflammation of the quadriceps tendon (tendinitis) can occur in people who run, bicycle or ski. It can also result from inflammatory diseases that occur throughout your body, most notably rheumatoid arthritis. Middle-age weekend warriors are more likely to rupture their quadriceps tendon than seasoned athletes are. And patellar tendon ruptures frequently occur in active younger people who have a history of tendinitis or who have had steroid injections in their knees.
        Meniscus injuries. A meniscus tear can result from aggressive pivoting or sudden turns - any activity that twists or rotates your knee. Occasionally, you can tear your meniscus while lifting something heavy. Older adults sometimes tear their meniscus during repetitive movements such as kneeling or squatting, but more often it tears because it has degenerated over time.
        Bursitis. Sometimes called housemaid's knee or carpet layer's knee, prepatellar bursitis often occurs after an activity that requires you to kneel for long periods - scrubbing floors, gardening or installing tile or carpet, for example. It can also result from an infection or as one of the symptoms of arthritis or gout.
        Dislocated kneecap. Kneecap (patellar) dislocations can occur in contact sports and in activities that require you to change direction while running, such as tennis, racquetball and volleyball. If your knees tend to turn inward or your kneecaps are higher than normal, you may be more prone to this injury.
        Osgood-Schlatter disease. This condition can develop in athletic young people during the years when their bones are growing rapidly - usually ages 10 to 15 for boys and 8 to 13 for girls. Osgood-Schlatter disease results from repeated tugging of the patellar tendon on a growth plate at the top of the tibia. This is most likely to occur during activities that involve running, jumping and bending, when the pull of the quadriceps muscle puts tension on the patellar tendon. In time, the tendon may begin to pull away from the tibia, resulting in a small bump you can see. In severe cases, the tendon may come away from the tibia completely.
        Hyperextended knee. This usually results from an awkward landing after a jump or from a contact injury.
        Iliotibial band syndrome. This is a common cause of lateral knee pain in runners. Competitive runners are especially susceptible, but amateurs aren't exempt. You're more likely to develop iliotibial band syndrome if you have biomechanical problems such as unequal leg length or weak hip abductors, the muscles responsible for sideways leg motion. Exercising on concrete surfaces or uneven ground, increasing the intensity or duration of your exercise too quickly, wearing worn or ill-fitting shoes, and excessive uphill or downhill running also can contribute to iliotibial band syndrome.

        Overuse. Any repetitive activity, from cycling a few miles every morning to gardening all weekend, can fatigue the muscles around your joints and lead to excessive loading stress. This causes an inflammatory response that damages tissue. If you don't allow your body time to recover, the cycle of inflammation and microdamage continues, putting you at increased risk of injury. It's not repeated motion itself that's to blame, but rather the lack of adequate recovery time. That's why current strength training guidelines advise against working the same muscle group on consecutive days, for example.
        Lack of muscle flexibility or strength. According to experts, lack of strength and flexibility are among the leading causes of knee injuries. Tight or weak muscles offer less support for your knee because they don't absorb enough of the stress exerted on your knee joints.
        Mechanical problems. Certain structural abnormalities, such as having one leg shorter than the other, misaligned knees, and even flat feet can make you more prone to knee problems.
        High-risk sports and activities. Some sports and activities put greater stress on your knees than others. Alpine skiing with its sharp twists and turns and potential for falls; basketball's jumps and pivots; and the repeated pounding your knees take when you run or jog all increase your risk of injury.
        Previous injury. Having a previous knee injury makes it more likely that you'll injure your knee again.
        Age. Certain types of knee problems are more common in young people - Osgood-Schlatter disease and patellar tendinitis, for example. Others, such as osteoarthritis, gout and pseudogout, tend to affect older adults.
        Sex. For reasons that aren't entirely clear, your sex may increase your risk of some types of knee injuries. Women are more prone to ACL tears than men are, and teenage girls are more likely than boys to experience a dislocated kneecap. Boys, on the other hand, are at greater risk of Osgood-Schlatter disease and patellar tendinitis than girls are.

        Can't bear weight on your knee
        Have significant knee swelling
        See an obvious deformity in your leg or knee
        Have significant pain
        Have a fever, in addition to redness, pain and swelling in your knee, which may indicate an infection

        X-ray. Your doctor may first recommend having an X-ray, which can help detect bone fractures and degenerative joint disease.
        Computerized tomography (CT) scan. This specialized X-ray, which creates cross-sectional images of the inside of your body, may help diagnose bone problems and detect loose bodies.